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Synthesis and Electromagnetic, Microwave Absorbing Properties of CoreShell Fe3O4Poly(3, 4-ethylenedioxythiophene) Microspheres Wencai Zhou,†,‡ Xiujie Hu,*,† Xiaoxia Bai,†,‡ Shuyun Zhou,*,† Chenghua Sun,† Jun Yan,† and Ping Chen† †
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, China ABSTRACT:
Highly regulated coreshell Fe3O4poly(3, 4-ethylenedioxythiophene) (PEDOT) microspheres were successfully synthesized by a two-step method in the presence of polyvinyl alcohol (PVA) and p-toluenesulfonic acid (p-TSA). And their morphology, microstructure, electromagnetic and microwave absorbing properties were subsequently characterized. By simply adjusting the molar ratio of 3, 4-ethylenedioxythiophene (EDOT) to Fe3O4 (represented by (EDOT)/(Fe3O4)), the thickness of the polymer shell can be tuned from tens to hundreds of nanometers. Moreover, it was found that the composite exhibited excellent microwave absorbing property with a minimum reflection loss (RL) of about 30 dB at 9.5 GHz with a (EDOT)/(Fe3O4) ratio of 20. KEYWORDS: Fe3O4PEDOT composite, coreshell, electromagnetic, two-step method, microwave absorbing, reflection loss
’ INTRODUCTION Recently, the conjugation of conducting polymers and inorganic magnetic nanoparticles has attracted more attention because the resultant materials not only exhibit a combination of the conductive and magnetic properties but also take the advantages of both nanomaterials and polymers. Besides, the inorganic magnetic nanoparticles coated by the conducting polymers will be prevented from reuniting caused by high surface activity. Therefore, these conductive and magnetic composites have great potential applications in the fields of electrical and magnetic shields, molecular electronics, nonlinear optics and microwave absorbing materials.14 Among magnetic metal oxides, Fe3O4 with properties of superparamagnetism in addition to its low toxicity and high biocompatibility is the most-studied material for magnetic nanoparticles5 in magnetic storagemedia, contrast agents for magnetic resonance imaging (MRI),6 separation of biomolecules,7 environmental or food analyzes,8 immunoassays,9 controlling targeted drug delivery/release,10 and microwave absorbing.11 Up to now, the researches on the fabrication of conductive and magnetic composites base on Fe3O4 are mainly focused on Fe3O4 polypyrrole/polyaniline (Fe3O4PPy/PANI). Liu et al. synthesized electric and ferromagnetic Fe3O4PPy composites by a r 2011 American Chemical Society
chemical method using p-dodecylbenzene sulfonic acid sodium salt (NaDS) as surfactant and dopant.12 Deng et al. prepared coreshell Fe3O4PPy nanoparticles and demonstrated that both the conductivity and the magnetization of the composites strongly depended on the Fe3O4 content and the doping degree.13 Lu et al. also synthesized highly regulated coreshell Fe3O4PPy microspheres with low conductivity.14 On the other hand, Reddy et al. synthesized electromagnetic functionalized Fe3O4 PANI composites with ammonium peroxydisulfate as the oxidizing agent.15 Poly (3,4-ethylenedioxythiophene) (PEDOT), a polythiophene derivative, is one of the most promising conductive polymers with excellent electrochemical activity, high electrical conductivity, moderate band gap, low redox potential, and excellent environmental stability. And in our recent study, we have revealed the microwave absorbing ability of PEDOT.16 Therefore, the composite consisted of Fe3O4 and PEDOT will have an attractive prospect. Reddy et al. have synthesized core shell nanocomposite composed of Fe3O4 nanoparticles and Received: April 18, 2011 Accepted: September 13, 2011 Published: September 13, 2011 3839
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PEDOT with lignosulfonic acid (LSA) serving as both the surfactant and the dopant.17 Even so, the preparation of highly regulated coreshell Fe3O4PEDOT micro- or nanospheres is still a challenge, which was rarely reported. Here, we propose a two-step method for the preparation of Fe3O4 microspheres with an integrated PEDOT shell. Fe3O4 hollow microspheres were first synthesized by solvothermal method,18 which has the advantage of lightweight in microwave absorbing, followed by EDOT polymerization on the microsphere surface. By simply adjusting the molar ratio of 3, 4-ethylenedioxythiophene (EDOT) to Fe3O4 (represented by (EDOT)/(Fe3O4)), the shell thickness of the composite, hence the electrical and magnetic properties, could be tuned. During the formation, polyvinyl alcohol (PVA) was used as stabilizer while p-toluenesulfonic acid (p-TSA) served as the dopant. The obtained composites exhibited excellent microwave absorbing property at appropriate shell thickness, with the maximum microwave absorbing reached about 30 dB at 9.5 GHz.
’ EXPERIMENTAL SECTION All reagents were used directly as received unless otherwise mentioned. Synthesis of Hollow Fe3O4 Microspheres. Fe3O4 hollow microspheres were synthesized according to literature:18 FeCl3 3 6H2O (2 mmol) and urea (6 mmol) were first dissolved in absolute ethylene glycol (25 mL). Then the solution was loaded into a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 200 °C for a definite time. After cooling to room temperature naturally, the black products were filtered off, washed with distilled water and absolute ethanol several times, and dried under a vacuum at 60 °C for 20 h. Synthesis of Fe3O4PEDOT Composites. The as-prepared hollow Fe3O4 microspheres were immersed in PVA (0.4 g) aqueous solution with ultrasonic for 20 min. p-TSA (20 mmol) was then added to the above mixture. The mixture was stirred for 20 min, and then EDOT (1 mmol) was introduced with stirring for 24 h. At last, ammonium persulfate (APS) (1 mmol) was added to prepare 100 mL of the mixture. After being stirred for 24 h at room temperature, the mixture was centrifuged and washed three times with a solvent of deionized water/ ethanol (1/1, v/v). The precipitate was dried under a vacuum at 60 °C for 24 h. Measurements. The morphology of the products was investigated using JEM-2100F transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectra in the range of 5002000 cm1 were conducted on sample pellets with KBr by means of an infrared spectrophotometer (Excalibur 3100, America, Varian).The phase identification of the fine powder composite was performed using X-ray diffraction (XRD) analysis on a D8 Focus Diffractometer (Germany). Conductivity measurements of the Fe3O4PEDOT samples (compressed into rectangular block) were performed using a Keithley 220 Source Meter fourpoint probe instrument. Magnetic properties were tested by vibrating sample magnetometer (VSM, Lakeshore 707 Series). The composites samples for electromagnetic parameter measurement were prepared by mixing the Fe3O4PEDOT microspheres and paraffin wax at different volume fraction of the Fe3O4PEDOT microspheres. The mixture was then pressed into a toroidal shape with the thickness of 2 mm. Subsequently, the relative complex permeability (μr) and permittivity (εr) were carried out by a HP8722ES network analyzer at the frequency range of 218 GHz and the reflection losses were calculated using the measured μr and εr.
’ RESULTS AND DISCUSSION Characterizations. Figure 1a shows the morphology of Fe3O4 hollow microspheres. The particles are spherical with diameters
Figure 1. TEM images of (a) pure Fe3O4 microspheres and (bf) Fe3O4PEDOT coreshell microspheres prepared with different (EDOT)/(Fe3O4) ratios: (b) 10, (c) 15, (d) 20, (e) 30, and (f) 50.
ranging from 200400 nm and a wall thickness of about 50 nm. The density of the prepared Fe3O4 microspheres is 3.56 g cm3 because of the existence of a hollow cavity, which is lower than that of the bulk Fe3O4. Figure 1bf shows the morphology of the obtained Fe3O4PEDOT coreshell microspheres prepared with different (EDOT)/(Fe3O4) ratios. It is clear that the Fe3O4 microspheres are fully coated by PEDOT and the shell gradually thickens with the increase of the (EDOT)/(Fe3O4) ratio. When (EDOT)/(Fe3O4) ratio is 10, the shell thickness is about 60 nm (Figure 1b). The shells increase to about 90 and 140 nm when (EDOT)/(Fe3O4) ratios are at 15 (Figure 1c) and 20 (Figure 1d), respectively. When (EDOT)/(Fe3O4) ratio reaches 30, the shell thickness can reach 250 nm (Figure 1e). Further, the shell thickness increases little when the (EDOT)/ (Fe3O4) ratio continues to increase to 50 (Figure 1f). There will be a saturation shell thickness if the (EDOT)/(Fe3O4) ratio continuously increases. The above results indicate that the (EDOT)/ (Fe3O4) ratio has a significant influence on the structure of the Fe3O4PEDOT coreshell microspheres. To identify the components of the composites, especially the polymer composition, we performed FTIR analyses of Fe3O4, pure PEDOT, and Fe3O4PEDOT composites prepared with (EDOT)/(Fe3O4) ratios of 10, 20, and 50. The spectra are shown in Figure 2. The FTIR spectrum of Fe3O4 (Figure 2A) 3840
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Scheme 1. Formation Mechanism of Fe3O4PEDOT Core Shell Microspheres
Figure 2. FTIR spectra of (A) Fe3O4; (BE) Fe3O4PEDOT composites prepared with different (EDOT)/(Fe3O4) ratios: (B) 10, (C) 20, (D) 50; and (E) pure PEDOT.
Figure 3. XRD patterns of (A) Fe3O4 microspheres and (BF) Fe3O4PEDOT composites prepared with different (EDOT)/ (Fe3O4) ratios: (B) 10, (C) 15, (D) 20, (E) 30, and (F) 50.
shows characteristic peak at 588 cm1, attributed to the FeO bond stretching. This peak shifts to 576 cm1 in the Fe3O4 PEDOT samples with a (EDOT)/(Fe3O4) ratio of 10 and continuously shifts to lower wavenumber and overlaps the peak of PEDOT with the increase of the (EDOT)/(Fe3O4) ratio. Figure 2E shows the spectra of pure PEDOT. The peaks at 690, 845, 922, and 983 cm1 are attributed to the deformation modes of CSC in the thiophene ring; the peaks at 1091, 1147, and 1203 cm1 are associated with the COC bending vibration of the ethylenedioxy moiety; the peak at 1357 cm1 is assigned to CC stretching of the quinoidal structure; the peaks at 1473 and 1517 cm1 are due to the CdC stretching of the quinoid structure of the thiophene ring. The main peaks of PEDOT shifts to high wavenumber with the increase of Fe3O4 in the composites compared to the pure PEDOT, which is due to some interaction of ferrite particles and polymer chains.19 Above all, the FTIR spectra confirm the coexistence of Fe3O4 and PEDOT. XRD patterns of Fe3O4 microspheres and Fe3O4PEDOT composites prepared with different (EDOT)/(Fe3O4) ratios were also observed (Figure 3). Fe3O4 (Figure 3A) shows diffraction peaks at 2θ = 18.4, 30.1, 35.6, 37.2, 43.1, 53.5, 57.1, and 62.7°, which are in agreement with literatures.18,20 These
peaks correspond to the (111), (220), (311), (222), (400), (422), (511), and (440) lattice planes. When (EDOT)/(Fe3O4) ratios are 10 (Figure 3B) and 15 (Figure 3C), the diffraction peaks of Fe3O4PEDOT composites are at the same position as the Fe3O4 microspheres (Figure 3A). However, with the (EDOT)/ (Fe3O4) ratios increasing to 20 (Figure 3D) and 30 (Figure 3E), new peaks at 2θ = 25.6 and 11.7° appear. These two peaks become stronger with higher (EDOT)/(Fe3O4) ratios, accompanied with decreasing intensity of the Fe3O4 peaks. The peaks at 2θ = 25.6 and 11.7° prove the existence of PEDOT according to literature.17 The diffraction patterns indicate that Fe3O4PEDOT composites are composed of pure phase with no impurity. Formation Mechanism. We explored the formation mechanism of the Fe3O4PEDOT coreshell structure by a series of experiments. It was found that the Fe3O4PEDOT coreshell structure could not form in the absence of PVA or p-TSA in the experiments. Without the inclusion of PVA, only a mixture of Fe3O4 microspheres and PEDOT was obtained. Most of PEDOT presented in amorphous state whereas a little polymer was found coated on the Fe3O4 microspheres to form a thin layer. There were Fe3O4 microspheres and a little polymer without pTSA, and the polymer aggregated apart from the Fe3O4 microspheres. Overall, the coexistent of PVA and p-TSA is important during the formation of Fe3O4PEDOT composites. In addition, we used Fe (p-toluene sulfonate) to replace APS and p-TSA in order to explore whether the oxidant with SO3 could act as the dual role of oxidant and dopant. The system did not form coreshell structure, which indicated that SO3 could not take the place of p-TSA. Then we suggest a possible mechanism of the Fe3O4PEDOT coreshell microspheres formation shown in Scheme 1. Fe3O4 particles are naturally hydrophilic due to plentiful hydroxyls on the particle surface.21 The hydroxyl group in PVA can form hydrogen bonds with the hydroxyl group on Fe3O4 particles, which enables Fe3O4 particles to be well dispersed. Because of the weak static interactions between the SO3 group in p-TSA molecules and Fe3O4 particles,14 p-TSA molecules can be absorbed on the surface of Fe3O4 particles. After EDOT monomer is added, the molecule tends to gather around the hydrophobic in PVA because of “similar compatibility”. Subsequently, p-TSA serves as the dopant to enhance the protonation of EDOT, thus connects EDOT to the Fe3O4 particles. Once APS oxidant is introduced, the polymerization will occur and EDOT monomer will be nucleated on the surface of Fe3O4 microspheres. After EDOT nucleation occurring on the surface of Fe3O4 microspheres, the polymerization will continue to carry out with the as-formed PEDOT. During the formation, PVA as 3841
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Figure 4. TEM images of Fe3O4PEDOT composite ((EDOT)/ (Fe3O4) = 10) with different polymerization time: (a) 24, (b) 48, and (c) 60 h.
Table 1. Electrical and Magnetic Properties of Fe3O4PEDOT composites composites with different
a
conductivity 1
Ms a 1
Mr b
Hcc
1
(EDOT)/(Fe3O4)
(S cm )
(emu g )
pure Fe3O4 10
5.28 104
84.8 58.8
7.6 3.6
61.9 64.7
15
1.21 103
40.3
2.2
58.8
20
1.06 102
17
0.82
59.1
30
2.34 101
11.3
0.56
61.1
50
3.13 101
0.38
78.1
5.85
(emu 3 g ) (Oe)
Saturation magnetization. b Remnant magnetization. c Coercivity.
the stabilizer, promotes the “oriented attachment”22 to join the as-formed PEDOT and gives rise to the shell. However, in the case of Fe (p-toluene sulfonate) replacing APS and p-TSA, the polymer shell did not form. This is because although the SO3 group in the Fe (p-toluene sulfonate) helped the attraction on the surface of Fe3O4 particles, it could not enhance the protonation of EDOT. That is to say, there is no link between Fe3O4 and EDOT, so the system could not form the coreshell structure. To prove the mechanism further, the other stabilizer and dopants were used to prepare Fe3O4PEDOT coreshell microspheres. It is found that polyvinyl pyrrolidone (PVP) as a typical stabilier can completely replace PVA. And β-naphthalenesulfonic acid (β-NSA) instead of p-TSA in Fe3O4PEDOT formation can form coreshell structure too, though the coreshell structure was not as good as that using p-TSA as the dopant. The carbonyl group in PVP can form a hydrogen bond with the hydroxyl group on the surface of Fe3O4 particles. Thus PVP can stabilize the polymer sols, and improve the dispersion of the particles because of steric hindrance effecting from PVP adsorption on particle surface.21 However, using oxalic acid as the dopant cannot produce Fe3O4PEDOT coreshell microspheres. The results indicate the interaction between SO3 group and Fe3O4 microspheres plays an important role in the formation of Fe3O4 PEDOT coreshell structure. In summary, the stabilizer and sulfonic acid group together promote the formation of core shell Fe3O4PEDOT microsphere. According to the suggested mechanism, EDOT monomer on the surface of Fe3O4 particles will increase and the organic layer will thicken as increase of the polymerization time. Figure 4 provides the evidence for this suppose. At a fixed (EDOT)/ (Fe3O4) ratio of 10, the shell of Fe3O4PEDOT microsphere increases from 60 to 100 nm following the reaction time increasing from 24 h (Figure 4a) to 48 h (Figure 4b). The shell
Figure 5. Magnetization curves applied magnetic field at room temperature of Fe3O4 microspheres and Fe3O4PEDOT composites prepared with different (EDOT)/(Fe3O4) ratios.
increase becomes minor after a reaction time longer than 60 h (Figure 4c). Electric and Magnetic Properties. The electrical properties of the obtained Fe3O4PEDOT composites were measured by four-point probe method and the conductivities are displayed in Table 1. It is found that the conductivities of the composites at room temperature are in the range of 1 104 to 1 101 S cm1 and increase with the (EDOT)/(Fe3O4) ratio increasing. The tendency is consistent with the shell thickness (Figure 1bf) because the conductivity is mainly determined by the polymer. The thicker the shell is, the higher the conductivity is. The magnetic properties of Fe3O4 and Fe3O4PEDOT microspheres prepared with different (EDOT)/(Fe3O4) ratios were investigated with a VSM which features a sensitivity of 1 105 emu. And Figure 5 shows the hysteresis loops of the obtained samples in the applied magnetic field sweeping from 10 to 10 kOe at room temperature. The magnetic parameters corresponding to Figure 5 are shown in Table 1. The pure Fe3O4 is a typical superparamagnetic material, presenting high saturation magnetization (Ms), high remnant magnetization (Mr), and low coercivity (Hc). With the increase of the (EDOT)/(Fe3O4) ratio, the saturation magnetization and remnant magnetization are decreased, due to the decrease of Fe3O4 content in the composites. The independence of coercivity on the (EDOT)/(Fe3O4) ratio suggests that the superparamagnetism has the same origin, from Fe3O4. It is clear from Table 1 that the Fe3O4PEDOT composites exhibit good magnetic properties and low conductivities with lower (EDOT)/(Fe3O4) ratios, while low magnetic properties and high conductivities with higher (EDOT)/(Fe3O4) ratios. Microwave Absorbing Properties. For a microwave-absorbing layer terminated by a short circuit, the normalized input impedance is related to the impedance in free space, Zin, and reflection loss (RL) is related to the normal incident plane wave, which can be given by the theory of the absorbing wall.23 Zin ¼
rffiffiffiffi μr 2πpffiffiffiffiffiffiffiffi μr εr fd tanh½ j c εr
Z 1 in RL ðdBÞ ¼ 20log Zin þ 1 3842
ð1Þ
ð2Þ
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Figure 6. Electromagnetic parameters of Fe3O4PEDOT composites with different (EDOT)/(Fe3O4) ratios at 50% volume fraction in the 218 GHz range: (a) real and (b) imaginary parts of the relative complex permittivity; (c) real and (d) imaginary parts of the relative complex permeability.
where c is the velocity of light in free space, f is the frequency, and d is the layer thickness. The relative complex permittivity (εr) and relative permeability (μr) of the absorbing medium are expressed as εr = ε0 jε00 , μr = μ0 jμ00 . The impedance matching condition is determined by the combination of the six parameters: ε0 , ε00 , μ0 , μ00 , f, and d. The real permittivity (ε0 ) and real permeability (μ0 ) symbolize the storage ability of electromagnetic energy,24 while the imaginary permittivity (ε00 ) is related to the dissipation of energy and the magnetic loss is expressed by imaginary permeability (μ00 ).19The curve of ε0 , ε00 , μ0 , and μ00 of Fe3O4PEDOT composites with different (EDOT)/(Fe3O4) ratios at 50% volume fraction are shown as Figure 6. It is observed that the samples with higher (EDOT)/(Fe3O4) ratios show higher values of ε0 and ε00 (Figure 6a,b), which is related to higher conductivities. Moreover, the μ0 values obviously decrease and then increase with the frequency increasing in the 218 GHz range (Figure 6c). When (EDOT)/(Fe3O4) ratios are 10 and 15, the μ00 values exhibit positive in the whole range; while a negative μ00 value means the magnetic energy is radiated out with no absorption.24 That is to say, the composites mainly exhibit electrical losses when (EDOT)/(Fe3O4) ratios are 20, 30, and 50. On the basis of formulas 1 and 2, we calculated the RL of Fe3O4PEDOT composites with different (EDOT)/(Fe3O4) ratios in the frequency range of 218 GHz at 50% volume
Figure 7. Reflection losses in the thickness of 2 mm of the Fe3O4 PEDOT composites prepared with different (EDOT)/(Fe3O4) ratios at 50% volume fraction.
fraction. Figure 7 shows the RL variation when the layer thickness is 2 mm. With a (EDOT)/(Fe3O4) ratio of 20, the minimum RL of the composite is 27.6 dB, which is better than pure PEDOT in our research before (24 dB).16 The minimum 3843
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Figure 8. Reflection losses in different thickness of Fe3O4PEDOT composites with (a) (EDOT)/(Fe3O4) = 20 and (b) 50 at 20% volume fraction (c) (EDOT)/(Fe3O4) = 20 and (d) 50 at 50% volume fraction.
reflection losses of the specimen with the (EDOT)/(Fe3O4) = 30 and 50 are 4.6 dB and 3.8 dB, respectively. Comparing with the conductivities in Table 1, it can be found that the composites with higher conductivities at (EDOT)/(Fe3O4) = 30 and 50 do not correspond to better absorbing parameters. This indicates that both higher conductivity and lower conductivity ((EDOT)/(Fe3O4) = 10) are not beneficial to improving microwave absorbing and the maximum microwave absorbing corresponds to an intermediate conductivity.25,26 In addition, to study the influence of volume fraction on microwave absorbing property, the electromagnetic parameters of Fe3O4PEDOT composites with (EDOT)/(Fe3O4) = 20 and 50 at 20% volume fraction were measured and the calculated reflection losses are shown as panels a and b in Figure 8, respectively. Meanwhile, panels c and d in Figure 8 show the calculated reflection losses of Fe3O4PEDOT composites with (EDOT)/(Fe3O4) = 20 and 50 at 50% volume fraction, respectively. When the volume fraction is 20% (Figure 8a), the sample with (EDOT)/(Fe3O4) = 20 exhibits excellent microwave absorbing property in the layer thickness range of 3- 4 mm and the minimum RL is 30 dB at 9.5 GHz with a layer thickness of 4 mm; when the volume fraction is 50% (Figure 8c), this composite exhibits good microwave absorbing property in the layer thickness range of 24 mm and the minimum RL is 27.6 dB at 13 GHz with a layer thickness of 2 mm. Besides, the RL of the Fe3O4PEDOT composite with (EDOT)/(Fe3O4) = 50 at 20% volume fraction (Figure 8b) is larger than the sample at 50%
volume fraction (Figure 8d) and the minimum RL is 22 dB at 18 GHz with a layer thickness of 2 mm. The result indicates that the conductivity, volume fraction and layer thickness all have great impacts on microwave absorbing property.
’ CONCLUSIONS Uniform coreshell Fe3O4PEDOT microspheres were successfully synthesized by a two-step method. The selection of both stabilizer and dopant are essential for the formation of the composites. The properties of the composites are significantly influenced by the (EDOT)/(Fe3O4) ratio. The Fe3O4PEDOT composites exhibited good conductivities at high (EDOT)/ (Fe3O4) ratios and excellent magnetic properties at low (EDOT)/ (Fe3O4) ratios. The reflection losses calculated by the theory of the absorbing wall showed that the Fe3O4PEDOT composite with (EDOT)/(Fe3O4) = 20 exhibited the best microwave absorbing property in the range of 218 GHz. The minimum RL reached approximated 30 dB at the thickness of 4 mm. In summary, the two-step synthesis and electromagnetic coreshell Fe3O4PEDOT composites will have a promising application in microwave absorbing field. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (X.H.);
[email protected]. ac.cn (S.Z.). Fax: +86 010-82543517; Tel: +86 010-82543515. 3844
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’ ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (20874112 and 60808022). The authors thank X. Huang and M. Wang for the assistance in TEM characterization. ’ REFERENCES (1) Kawaguchi, H. Prog. Polym. Sci. 2000, 25, 1171–1210. (2) Zhang, L.; Wan, M. J. Phys. Chem. B 2003, 107, 6748–6753. (3) Kurlyandskaya, G. V.; Cunanan, J.; Bhagat, S. M.; Aphesteguy, J. C.; Jacobo, S. E. J. Phys. Chem. Solids 2007, 68, 1527–1532. (4) Marchessault, R. H.; Rioux, P.; Raymond, L. Polymer 1992, 33, 4024–4028. (5) Lu, W.; Shen, Y.; Xie, A.; Zhang, W. J. Magn. Magn. Mater. 2010, 322, 1828–1833. (6) Hu, F. Q.; Wei, L.; Zhou, Z.; Ran, Y. L.; Li, Z.; Gao, M. Y. Adv. Mater. 2006, 18, 2553–2556. (7) Seino, S.; Kinoshita, T.; Otome, Y.; Nakagawa, T.; Okitsu, K.; Nakayama, T.; Sekino, T.; Niihara, K.; Yamamoto, T. A. J. Ceram. Process. Res. 2004, 5, 136–139. (8) Rana, S.; White, P.; Bradley, M. Tetrahedron Lett. 1999, 40, 8137–8140. (9) Kuramitz, H. Anal. Bioanal. Chem. 2009, 394, 61–69. (10) Yang, X.; Zhang, X.; Ma, Y.; Huang, Y.; Wang, Y.; Chen, Y. J. Mater. Chem. 2009, 19, 2710–2714. (11) Ni, S.; Wang, X.; Zhou, G.; Yang, F.; Wang, J.; He, D. J. Alloys Compd. 2010, 489, 252–256. (12) Liu, J.; Wan, M. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2734–2739. (13) Deng, J.; Peng, Y.; He, C.; Long, X.; Li, P.; Chan, A. S. C. Polym. Int. 2003, 52, 1182–1187. (14) Lu, X.; Mao, H.; Zhang, W. Polym. Compos. 2009, 30, 847–854. (15) Reddy, K. R.; Lee, K. P.; Gopalan, A. I. Colloids. Surf., A 2008, 320, 49–56. (16) Ni, X.; Hu, X.; Zhou, S.; Sun, C.; Bai, X.; Chen, P. Polym. Adv. Technol. 2011, 22, 532–537. (17) Reddy, K. R.; Park, W.; Sin, B. C.; Noh, J.; Lee, Y. J. Colloid Interface Sci. 2009, 335, 34–39. (18) Lv, Q. R.; Fang, Q. Q.; Liu, Y. M.; Wang, W. N. Chin. J. Phys. 2010, 48, 417–423. (19) Ohlan, A.; Singh, K.; Chandra, A.; Dhawan, S. K. ACS Appl. Mat. Interfaces 2010, 2, 927–933. (20) Hu, P.; Yu, L.; Zuo, A.; Guo, C.; Yuan, F. J. Phys. Chem. C 2008, 113, 900–906. (21) Guo, L.; Pei, G.-L.; Wang, T.-J.; Wang, Z.-W.; Jin, Y. Colloids Surf., A 2007, 293, 58–62. (22) Yang, Q.; Tang, K.; Wang, C.; Qian, Y.; Zhang, S. J. Phys. Chem. B 2002, 106, 9227–9230. (23) Abbas, S. M.; Dixit, A. K.; Chatterjee, R.; Goel, T. C. Mater. Sci. Eng., B 2005, 123, 167–171. (24) Wang, C.; Han, X.; Xu, P.; Wang, J.; Du, Y.; Wang, X.; Qin, W.; Zhang, T. J. Phys. Chem. C 2010, 114, 3196–3203. (25) Xu, P.; Han, X.; Wang, C.; Zhao, H.; Wang, J.; Wang, X.; Zhang, B. J. Phys. Chem. B 2008, 112, 2775–2781. (26) Unsworth, J.; Kaynak, A.; Lunn, B.; Beard, G. J. Mater. Sci. 1993, 28, 3307–3312.
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